SUMMARY The mucus covering our mucosal surfaces is an intimate part of the innate immune system and the first line of defense against microbial challenges. This is especially prominent in the lower parts of the intestine where we have to protect ourselves at the same time as we live in a symbiotic relation without trigger an overt immune response. We have shown that colon has a double-layered mucus layer built around the MUC2 mucin. The inner of these act as a barrier and does not allow bacteria to penetrate. With an absence of MUC2 or defects in the mucus, bacteria reach the epithelial cells, penetrate into the crypts, and into the epithelial cells. In experimental colitis models this inner mucus layer is penetrable to bacteria and in patients with active colitis. This renewal is to continue studies on how the mucus layers are formed and built by the use of biochemical methods, especially mass spectrometry, and the use of various types of gene knock-out animals that are colonized with bacteria or germ-free. Patient studies on biopsies will follow changes in penetrability processes over the cyclic changes observed ulcerative colitis. The role of different mucus proteins on mucus properties will be studied and manipulated by recombinant mucus proteins and pharmacological agents. The three aims that will be studied are: Aim 1: To obtain a deeper functional and molecular understanding of the small intestinal mucus and its main component the MUC2 mucin in relation to mucus detachment, bacterial selection, and goblet cell uptake of antigens via transglutamination. Aim 2. To obtain a functional and cell biological understanding of the many different types of goblet cells in the intestine and their different secretory machinery. Aim 3. To obtain a functional understanding of how the mucus and its main components protect the mucosal surfaces of colon and inhibits ulcerative colitis. Expected results are novel ways to improve the protection of colon and by novel principles of the intestinal mucosal immunology that could have importance for understanding the disease ulcerative colitis.